To determine if cytomegalovirus (CMV) retinitis occurs more frequently in patients infected with certain strains CMV isolates from the blood of 44 patients with advanced human immunodeficiency virus disease were grouped by the DNA sequence or the restriction endonuclease digest pattern of a portion of the glycoprotein B (gB) gene. Forty-two patients (95%) were followed clinically until the development of CMV retinitis or death. Fourteen (78%; 95% confidence interval, 7%-39%) of 26 with isolates belonging to other gB groups developed CMV retinitis (P = .002). Viremia caused by gB group 2 CMV strains is associated with higher risk of CMV retinitis than viremia due to other CMV gB groups. The association of CMV gB gene with retinitis suggests this gene, or one linked to it, is a virulence factor for CMV strains causing infection in AIDS patients.